An oil supplying screw compressor, for instance, which is one of liquid supplying air compressors, is provided with a compressor body that has a pair of male and female screw rotors and is configured to inject oil into a compression chamber for purposes of cooling the heat of compression, improving the sealability of the compression chamber, and lubrication of the screw rotors. Compressed air that has been compressed to a predetermined pressure in the compression chamber of the compressor body is delivered in a state of being mixed with the oil. After the oil has been separated from the compressed air by a separator, the resulting compressed air is supplied to a user's destination via a compressed air system. The separated oil is temporarily accumulated in a lower portion of the separator and then supplied to the compressor body via an oil system by an internal pressure of the separator. Namely, the oil circulates between the compressor body and the separator.
The air compressor adopts a scheme for controlling a capacity in response to a utilization state of the compressed air, with a view to power reduction. Specifically, for instance, an intake throttle valve is provided on an intake side of the compressor body, an air release system is provided to be connected to a primary side of a check valve in the compressed air system (in other words, a secondary side of the separator), and an air release valve is provided in this air release system. Furthermore, a pressure sensor is provided on a secondary side of the check valve in the compressed air system. For instance, when the quantity of the used compressed air falls and the pressure detected by the pressure sensor reaches a predetermined upper limit, a mode of the air compressor is switched to either a no-load operation mode or an automatic stop mode and the supply of the compressed air is stopped. In the no-load operation mode and the automatic stop mode, the following controls are exerted.
In the no-load operation mode, the intake throttle valve is closed while the compressor body continues to operate without stopping a motor. Furthermore, the air release valve is opened to release the compressed air, and the pressure on the primary side of the check valve in the compressed air system, that is, the internal pressure of the separator is reduced to some extent. Subsequently, when the pressure detected by the pressure sensor falls to a predetermined lower limit, the mode is switched to a load operation mode. In other words, the intake throttle valve is opened and the air release valve is closed.
In the automatic stop mode, the motor is stopped to stop the compressor body. Furthermore, the air release valve is opened to release the compressed air, and the pressure on the primary side of the check valve in the compressed air system, that is, the internal pressure of the separator is reduced nearly to an atmospheric pressure. Moreover, the intake throttle valve is closed in order to prevent the oil within the compressor body from flowing backward and then to the primary side of the intake throttle valve. Subsequently, when the pressure detected by the pressure sensor falls to the predetermined lower limit, the mode is switched to the load operation mode. In other words, the motor is driven to restart operation of the compressor body. Furthermore, the intake throttle valve is opened and the air release valve is closed.
In this case, if the time from stop to restart of the compressor body is short, the internal pressure of the separator falls insufficiently, and the residual pressure thereof causes starting congestion of the compressor body. Owing to this, a time limit from the stop of the compressor body until the compressor body becomes restartable is set, and the compressor body is restarted after this time limit has elapsed.
Note that Patent Document 1, for instance, uses an air-pressure-operated intake throttle valve and a solenoid-operated air release valve.